Marketable Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Debt securities	$ 1,713,866	$ 684,609
Equity securities	138,729	139,571
Total marketable investment securities	1,852,595	824,180
Less: Restricted marketable investment securities	9,720	10,019
Marketable investment securities, at fair value	1,842,875	814,161
Corporate bonds
Schedule of Investments [Line Items]
Debt securities	1,496,456	542,573
Other debt securities
Schedule of Investments [Line Items]
Debt securities	$ 217,410	$ 142,036

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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